Nav: Home

Two paths at once: Watching the buildup of quantum superpositions

November 10, 2016

It is definitely the most famous experiment in quantum physics: in the double slit experiment, a particle is fired onto a plate with two parallel slits, so there are two different paths on which the particle can reach the detector on the other side. Due to its quantum properties, the particle does not have to choose between these two possibilities, it can pass through both slits at the same time. Something quite similar can be observed when a helium atom is ionized with a laser beam.

Just like the two paths through the plate, the ionization of helium can happen via two different processes at the same time, and this leads to characteristic interference effects. In the case of the helium atom, they are called "Fano resonances". A team of scientists from TU Wien (Vienna, Austria), the Max-Planck Institute for Nuclear Physics in Heidelberg (Germany) and Kansas State University (USA) has now managed to observe the buildup up of these Fano resonances -- even though this effect takes place on a time scale of femtoseconds.

The experiment was performed in Heidelberg, the original proposal for such an experiment and computer simulations were developed by the team from Vienna, additional theoretical calculations came from Kansas State University.

Direct and Indirect Path

When a laser pulse transfers enough energy to one of the electrons in the helium atom, the electron is ripped out of the atom right away.

There is, however, another way to ionize the helium atom, which is a little bit more complex, as Professor Joachim Burgdörfer (TU Wien) explains: "If at first the laser lifts both electrons to a state of higher energy, one of the electrons may return into the state of lower energy. Part of this electron's energy is transferred to the second electron, which can then leave the helium atom."

The outcome of these two processes is exactly the same - both turn the neutral helium atom into an ion with one remaining electron. From this perspective, they are fundamentally indistinguishable.

Fano Resonances

"According to the laws of quantum physics, each atom can undergo both processes at the same time", says Renate Pazourek (TU Wien). "And this combination of paths leaves us characteristic traces that can be detected." Analyzing the light absorbed by the helium atoms, so-called Fano resonances are found - an unmistakable sign that the final state was reached via two different paths.

This can also be prevented. During the ionization process, the indirect path can be effectively switched off with a second laser beam so that only the other path remains open and the Fano-resonance disappears.

This opens up a new possibility of studying the time evolution of this process. At first, the atom is allowed to follow both paths simultaneously. After some time, the indirect path is blocked. Depending on how long the system was allowed to access both paths, the Fano-resonance becomes more or less distinct.

"Fano resonances have been observed in a wide variety of physical systems, they play an important role in atomic physics", says Stefan Donsa (TU Wien). "For the first time, it is now possible to control these resonances and to show precisely, how they build up within femtoseconds." "These quantum effects are so fast that on our usual time scales they appear to happen instantaneously, from one moment to the next", says Stefan Nagele. "Only by employing new sophisticated methods of attosecond physics it has become possible to study the time evolution of these processes."

This does not only help quantum scientists to understand the fundamental theory of important quantum effects, it also opens up new possibilities of controlling such processes -- for example facilitating or inhibiting chemical reactions.
Original publication: Observing the ultrafast build-up of a Fano resonance in the time domain, A. Kaldun et al., Science. (DOI: 10.1126/science.aah6972)

In the same issue of Science magazine, a team of scientists from France and Spain has published another paper, in which a complementary method of time-resolved photoelectron spectroscopy is used to obtain a view on the Fano resonance (DOI: 10.1126/science.aah5188).

Further information:

Prof. Joachim Burgdörfer
Institute for Theoretical Physics
TU Wien
Wiedner Hauptstraße 8-10, 1040 Vienna
T: +43-1-58801-13610

Dr. Stefan Nagele
Institute for Theoretical Physics
TU Wien
Wiedner Hauptstraße 8-10, 1040 Vienna
T: +43-1-58801-13607

Vienna University of Technology

Related Quantum Physics Articles:

Quantum physics: On the way to quantum networks
Physicists at Ludwig-Maximilians-Universitaet (LMU) in Munich, together with colleagues at Saarland University, have successfully demonstrated the transport of an entangled state between an atom and a photon via an optic fiber over a distance of up to 20 km -- thus setting a new record.
Quantum physics: Controlled experiment observes self-organized criticality
Researchers from Cologne, Heidelberg, Strasbourg and California have observed important characteristics of complex systems in a lab experiment.
A platform for stable quantum computing, a playground for exotic physics
Harvard University researchers have demonstrated the first material that can have both strongly correlated electron interactions and topological properties, which not only paves the way for more stable quantum computing but also an entirely new platform to explore the wild world of exotic physics.
A new quantum data classification protocol brings us nearer to a future 'quantum internet'
A new protocol created by researchers at the Universitat Autònoma de Barcelona sorts and classifies quantum data by the state in which they were prepared, with more efficiency than the equivalent classical algorithm.
Quantum physics: Ménage à trois photon-style
When two photons become entangled, the quantum state of the first will correlate perfectly with the quantum state of the second.
Quantum physics -- Simulating fundamental interactions with ultracold atoms
An international team of physicists succeeded in precisely engineering key ingredients to simulate a specific lattice gauge theory using ultracold atoms in optical lattices.
A key piece to understanding how quantum gravity affects low-energy physics
In a new study, led by researchers from SISSA (Scuola Internazionale Superiore di Studi Avanzati), the Complutense University of Madrid and the University of Waterloo, a solid theoretical framework is provided to discuss modifications to the Unruh effect caused by the microstructure of space-time.
Helping physics teachers who don't know physics
A shortage of high school physics teachers has led to teachers with little-to-no training taking over physics classrooms, reports show.
Quantum physics and origami for the ultimate get-well card
The bizarre optical properties of tiny metal particles -- smaller than light waves -- can be captured on paper to detect even a single target molecule in a test sample.
Can artificial intelligence solve the mysteries of quantum physics?
A new study published in Physical Review Letters by Prof.
More Quantum Physics News and Quantum Physics Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Climate Mindset
In the past few months, human beings have come together to fight a global threat. This hour, TED speakers explore how our response can be the catalyst to fight another global crisis: climate change. Guests include political strategist Tom Rivett-Carnac, diplomat Christiana Figueres, climate justice activist Xiye Bastida, and writer, illustrator, and artist Oliver Jeffers.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Speedy Beet
There are few musical moments more well-worn than the first four notes of Beethoven's Fifth Symphony. But in this short, we find out that Beethoven might have made a last-ditch effort to keep his music from ever feeling familiar, to keep pushing his listeners to a kind of psychological limit. Big thanks to our Brooklyn Philharmonic musicians: Deborah Buck and Suzy Perelman on violin, Arash Amini on cello, and Ah Ling Neu on viola. And check out The First Four Notes, Matthew Guerrieri's book on Beethoven's Fifth. Support Radiolab today at